Investigation of Low-temperature Solution-processed Thin-Film Transistors for Flexible Displays

This thesis describes the electrical behaviour of solution-processed zinc oxide thin film transistors (ZnO TFTs) fabricated at low temperature. First, the electrical properties of solution-processed ZnO films are reported. Spin-coated ZnO films annealed at 150 °C exhibit significant sensitivity to the ambient environment. However, their stability can be improved by hydrogen plasma treatment. Zinc oxide TFTs (channel width/length = 4000/200 μm) fabricated by chemical spray pyrolysis at the low process temperature of 140 °C are investigated. The resulting transistors exhibit a saturation mobility of 2 cm2/Vs measured in air; this value is reduced to 0.5 cm2/Vs under vacuum. The effect of hydrogen plasma treatment on spin-coated ZnO TFTs is then studied. The electrical characteristics of untreated TFTs exhibit large hysteresis and a positive threshold voltage shift on repeated measurements. These effects are reduced by the hydrogen plasma and an increase in carrier mobility is observed. In a further investigation, a solution-processed silicon dioxide gate insulator for application in the TFTs is used; a perhydropolysilazane (PHPS) precursor is spin-coated with subsequent thermal treatment to form the SiO2 layer. Exposure to oxygen plasma leads to an acceleration of the conversion reaction, resulting in good insulating properties (leakage current density of ~10-7 A/cm2) and TFT performance (channel width/length = 1000/50 μm, carrier mobility of 3.2 cm2/Vs, an on/off ratio of ~107, a threshold voltage of -1.3 V and a subthreshold swing of 0.2 V/decade). Finally, a photolithographic process is introduced for the fabrication of ‘short’ channel solution-processed ZnO TFTs. Optimum processing conditions are established and used for the fabrication of transistors having various channel dimensions. Devices with a minimum channel length of 5 μm possessed a mobility of 1.5 x 10-2 cm2/Vs, on/off ratio of 106 and good contact between the S/D electrodes and the semiconductor. The relatively low mobility could originate from gate insulator roughness caused by the photolithographic processes

Short Channel Effect of Solution-Processed ZnO Thin Film Transistors: Optimization for Photolithographic Process

A photolithographic process for the fabrication of short channel solution-processed zinc oxide transistors (ZnO TFTs) was optimized. To avoid damage to the ZnO film by the photolithography, a bottom gate, bottom contact (BG-BC) device structure was adopted. A perhydropolysilazane (PHPS) precursor, thermally annealed and then treated in oxygen plasma, was used as the gate insulator on an aluminum (Al) gate electrode. Al source and drain electrodes, with a minimum channel length of 5 μm were successfully defined using photolithography. A mobility of 1.5×10−2 cm2/Vs, on/off ratio of 106 and good contact between the source and drain (S/D) and semiconductor were achieved for solution-processed ZnO TFTs having various channel lengths, showing no degradation of device properties. The relatively low mobility was attributed to increased roughness of the gate insulator resulting from development during the photolithographic process. These results suggest that short-channel solution-processed ZnO TFTs can be fabricated by adopting optimised photolithographic processes

Current Transport Mechanism in Palladium Schottky Contact on Si-Based Freestanding GaN

In this study, the charge transport mechanism of Pd/Si-based FS-GaN Schottky diodes was investigated. A temperature-dependent current–voltage analysis revealed that the I-V characteristics of the diodes show a good rectifying behavior with a large ratio of 103–105 at the forward to reverse current at ±1 V. The interface states and non-interacting point defect complex between the Pd metal and FS-GaN crystals induced the inhomogeneity of the barrier height and large ideality factors. Furthermore, we revealed that the electronic conduction of the devices prefers the thermionic field emission (TFE) transport, not the thermionic emission (TE) model, over the entire measurement conditions. The investigation on deep level transient spectroscopy (DLTS) suggests that non-interacting point-defect-driven tunneling influences the charge transport. This investigation about charge transport paves the way to achieving next-generation optoelectronic applications using Si-based FS-GaN Schottky diodes

Development of boron doped diamond electrodes material for heavy metal ion sensor with high sensitivity and durability

We report on the optimized substrate pretreatment and deposition process conditions for boron-doped diamond (BDD) electrodes fabricated by hot-filament chemical vapor deposition (HFCVD). The optimized BDD electrode with a doping concentration of 8000 ppm showed high accuracy and precision in detecting Cd(II), Pb(II), and Cu(II) ions. In addition, this demonstrates excellent selectivity against external metal ions under the optimized stripping voltammetry measurement conditions. The detection limits of the target ions of Cd(II), Pb(II), and Cu(II) were 0.55 (+/- 0.05), 0.43 (+/- 0.04), and 0.74 (+/- 0.06) mg/L (S/N = 3), respectively. In real samples spiked with 100 mg/L Cd(II), Pb(II), and Cu(II), both the accuracy and precision of the BDD electrode were within 5%; the interference with organic matter was also negligible. The excellent selectivity and long-term stability indicate that the BDD electrode developed in this study are potentially useful for online water environment monitoring systems

Optimization of a Solution-Processed SiO2 Gate Insulator by Plasma Treatment for Zinc Oxide Thin Film Transistors

We report on the optimization of the plasma treatment conditions for a solution-processed silicon dioxide gate insulator for application in zinc oxide thin film transistors (TFTs). The SiO2 layer was formed by spin coating a perhydropolysilazane (PHPS) precursor. This thin film was subsequently thermally annealed, followed by exposure to an oxygen plasma, to form an insulating (leakage current density of ∼10−7 A/cm2) SiO2 layer. Optimized ZnO TFTs (40 W plasma treatment of the gate insulator for 10 s) possessed a carrier mobility of 3.2 cm2/(V s), an on/off ratio of ∼107, a threshold voltage of −1.3 V, and a subthreshold swing of 0.2 V/decade. In addition, long-term exposure (150 min) of the pre-annealed PHPS to the oxygen plasma enabled the maximum processing temperature to be reduced from 180 to 150 °C. The resulting ZnO TFT exhibited a carrier mobility of 1.3 cm2/(V s) and on/off ratio of ∼107

Optimization of a Solution-Processed SiO₂ Gate Insulator by Plasma Treatment for Zinc Oxide Thin Film Transistors

We report on the optimization of the plasma treatment conditions for a solution-processed silicon dioxide gate insulator for application in zinc oxide thin film transistors (TFTs). The SiO₂ layer was formed by spin coating a perhydropolysilazane (PHPS) precursor. This thin film was subsequently thermally annealed, followed by exposure to an oxygen plasma, to form an insulating (leakage current density of ∼10⁻⁷ A/cm²) SiO₂ layer. Optimized ZnO TFTs (40 W plasma treatment of the gate insulator for 10 s) possessed a carrier mobility of 3.2 cm²/(V s), an on/off ratio of ∼10⁷, a threshold voltage of −1.3 V, and a subthreshold swing of 0.2 V/decade. In addition, long-term exposure (150 min) of the pre-annealed PHPS to the oxygen plasma enabled the maximum processing temperature to be reduced from 180 to 150 °C. The resulting ZnO TFT exhibited a carrier mobility of 1.3 cm²/(V s) and on/off ratio of ∼10⁷